The ability to manipulate the mouse genome via ES cell technology has had a tremendous impact on our ability to identify the genes required for the development of mammalian embryos. Much is still to be understood about the cell-cell interactions necessary to establish cell lineages and to organize tissues. We propose to develop a binary transgenic system based on Cre/lox technology for ablating specific cell populations in mice. Using this system it will be possible to produce embryos with virtually any cell type ablated by mating two phenotypically normal mouse lines. One line (the "universal ablator") will have the potential to express a cytotoxin in eves cell, but cytotoxin will not be produced unless Cre enzyme is expressed. The other mouse line will express the cre gene in the intermediate mesoderm (IM). By mating these two lines, we will eliminate the IM. This will enable us to investigate the function of the IM in the early mouse embryo, and specifically to test the hypothesis that the IM produces the limb inducer. This system will potentially be useful for analyzing the functions of many different tissues in the regulation of normal and abnormal organ growth and development.